1. Field of the Invention
Aspects of the invention relate generally to optical communication systems, and more particularly to mitigation of polarization mode dispersion (PMD) of multiple optical communication channels.
2. Description of the Prior Art
Optical communication systems typically employ optical fibers for carrying optical communication signals over significant distances. These optical signals typically take the form of a series of light pulses carrying encoded voice information or digital data. As shown in the simplified diagram of FIG. 1, an optical communication system 1 typically includes an optical transmitter 2 and an optical receiver 4 coupled by way of an optical fiber 6. Using such a system 1, the optical transmitter 2 transforms an electrical communication signal 8 into an optical communication signal 10, which is sent to the optical receiver 4 over the optical fiber 6. The optical receiver 4 then converts the optical communication signal 10 back to a received electrical signal 12.
Like other forms of communication, the optical communication signal 10 of the optical communication system 1 is subject to various forms of noise or distortion, thus possibly reducing the fidelity of the optical signal 10 after passing along an optical fiber 6. One particular form of distortion of optical signals 10 is polarization mode dispersion (PMD). Oftentimes, due to manufacturing processes, mechanical stresses, and the like, the cross-sectional shape of the optical fiber 6 may become asymmetric. As a result, such a fiber 6 exhibits asymmetric light propagation characteristics that allow light propagating in a first plane of polarization to propagate more quickly than light propagating in a second plane of polarization perpendicular to the first. These two planes are normally referred to as the principal states of polarization (PSP), and the resulting time delay between the two PSPs is often referred to as differential group delay (DGD). As a result, pulses from the optical transmitter 2 at one end of an optical fiber 6 tend to disperse, or spread in time, by the moment they arrive at the optical receiver 4 at the opposing end of the fiber 6. Thus, each optical pulse may appear as two separate, but closely situated, optical pulses to the receiver 4. Also, adjacent optical pulses may begin to merge. In either case, the “eyes” of an eye pattern produced by the optical pulses tend to shrink or close completely, typically resulting in data corruption or loss at the receiving end.
The deleterious effects of PMD worsen as data rate increases. While PMD has proved to be somewhat problematic at data rates of 10 gigabits-per-second (10 Gbps, or 10 G), PMD has proven to be a major barrier to implementing newer 40 G systems being developed to increase optical communication bandwidth and capacity.
Some methods for mitigating PMD at an optical receiver 4 have been devised. Normally, such methods involve employing a compensator that separates the received optical signal into its two PSPs, typically by way of a polarization beam splitter. The compensator then delays the faster of the two PSPs by the exhibited DGD via a feedback controller to essentially negate the effects of the PMD at the receiver 4.
Unfortunately, the efficacy of PMD compensators is restricted in optical communication systems employing wavelength division multiplexing (WDM) or dense wavelength division multiplexing (DWDM). In such systems, multiple communication channels are carried over a single optical fiber by dividing the available bandwidth into several relatively narrow bandwidth or frequency ranges, with each range carrying a single communication channel. FIG. 2 illustrates in a simplified manner a typical WDM optical communication system 100 having a WDM optical transmitter 110, a WDM optical receiver 120, and an optical fiber 130. The optical transmitter 110 includes single-channel optical transmitters 112, each for translating an electrical communication signal 111 into an associated WDM communication channel signal 113 for a particular WDM channel. A WDM multiplexer 114 is used to combine the various WDM channel signals 113 into a combined optical communication signal 140 to be transferred over the fiber 130. Similarly, the optical receiver 120 includes a WDM demultiplexer 124 to separate the various received WDM channel signals 123 comprising the combined optical signal 140, and multiple single-channel receivers 122 for translating each received WDM optical channel signal 123 into a corresponding received electrical signal 121 carrying the communicated information.
As no PMD compensation is provided in the optical communication system 100, PMD exhibited by the fiber 130 is likely to adversely affect the quality of the received WDM optical signals 123 being processed by the optical receiver 120. In addition, the magnitude of the delaying effects of PMD is known to be wavelength-dependent. Thus, simultaneous PMD mitigation of all WDM channel signals 113, as embodied in the combined optical signal 140, by way of a single PMD compensator is normally ineffective. As a result, multiple compensators are typically required for PMD mitigation of the optical signal 140, thereby increasing the cost and complexity of the optical communication system 100.